204 Herbert L. Hawkins — On Lanieria of Duncan. 



and Gauth., from the Lower Cretaceous of Algeria, resembles Lanieria 

 in its arabulacral complexity, and is otherwise closely similar to 

 Coenholedypus. I have not seen a specimen, but the figure given by 

 Cotteau, Peron, and Gauthier {Ech. foss. Algerie) shows the ambu- 

 lacra to bo even more like those of Conulus than are those of Lanieria. 

 The adapical member of each triad, instead of being a thin lath with 

 parallel edges (as in Lanieria') is wedge-shaped, being pinched at its 

 l^erradial end. 



The systematic position of these two genera may be summarized as 

 follows : — 



HOLECTYPOIDA. DlSCOIDIID^. 



LANIEKIINiE. 



Two genera. Lanieria, Duncan. Li. lanicri, type. 



Discholectypus, Pomel. D. vieslei, type. 



5. The Biological Significancp: of the Laniehiin^. 



Assuming that the foregoing systematic association of the Lanieriinse 

 with the Discoidiidse is correct, an interesting feature in morphogenesis 

 is presented. The peculiar ambulacral structure of Conulus (which is 

 continued, probably by direct descent, in Echinoneus and Mtcropetalon), 

 although obviously connected with the usual structure of the 

 Holectypoida, is so definite that it would be thought to be a special 

 development not likely to be repeated independently. 



Excluding Amhli/pygus from the discussion (for this genus maj' 

 possibly be an Echinoneid, though apparently closely related to the 

 Echinolampidse), we find that ' Comdus structure ' has appeared three 

 times in the history of the Holectypoida. In one case, Discholectypus, 

 the ambulacral complexity is unaccompanied hy unj other similarity 

 to the Conulus type, while in Lanieria the general facies of the test 

 (although no otlier detail of structure) conforms to that of Conulus. 

 It seems impossible that Discholectypus and Lanieria can be in direct 

 phylogenetic sequence, taking into consideration their distant and 

 restricted localities and widely separate horizons. Further, the 

 nature of the ambulacral plating does not, in this case, seem at all to 

 be connected with any adaptive purpose. Except, perhaps, near the 

 peristome, there is no actual increase in the number of plates 

 (^i.e. jiodia) achieved by the 'plate-crushing'. 



In the independent development of this peculiar structure in at 

 least three distinct types of the Holectypoida we seem to find an 

 expression of some fundamental tendency in evolution which, within 

 tlie limits of an order, can cause one set of structures to be developed, 

 without regard to the others, on a definite plan. It maybe suggested 

 that there is an objective, or goal, towards which the evolution of 

 a group is tending, and which may be reached independently, and 

 piecemeal, by various members of the ramifying series of the group. 



Expressing this phenomenon in somewhat ' diagrammatic' language, 

 one may suggest that at the inception of an order the first member is 

 a complex association of parts in definite stages of development ; and 

 for each part there is a certain ideal line of evolution along which, as 

 far as possible, it will travel independently towards a certain limit. 

 The succeeding members of the order will be composed of the same 



